Formation damage due to invasion by drilling fluids is a well-known problem. Many zones contain formation clays, which hydrate when in contact with water, such as the filtrate from water-based drilling fluids. These hydrated clays tend to block the producing zones, primarily sands, so that oil and gas cannot move to the borehole and be produced. These zones are also damaged by solids, which are carried into the openings with the drilling fluid. The movement of drilling fluids and filtrate through these openings also causes dislodging and migration of solids in place in the formation. These solids can lodge and block movement of produced hydrocarbons.
Fluid invasion is caused by the differential pressure between the hydrostatic pressure and fluid viscosity (equivalent circulating density (ECD)) and the formation pressure; differential pressure is especially large in low pressure or depleted zones. The rate of invasion is controlled by the differential pressure, the fluid viscosity, the structure of the pore network in the rock and any fissures in the rock that may be present. Drillers have long used filtrate control mechanisms to reduce the movement of drilling fluids and filtrate into and through the formation openings. The mechanism generally involves the creation of a filter cake along the borehole wall. This technique still allows some fluid in and out of the zone. Although some fluid loss may be desirable to provide a favorable drilling rate, too high a fluid loss can result in costly mud bills and excessive cake thickness, which can lead to other problems, such as differential sticking of the drill string.
More recent technology has seen the development of Low Shear Rate Viscosity (LSRV) fluids. High LSRV is generated by the addition of specialized viscosifiers to water or brines to form a drilling fluid. These viscosifiers have a unique ability to create extremely high viscosity at very low shear rates. These LSRV fluids have been widely used because of their solids suspension ability. They have been accepted as a way to minimize cuttings bed formation in high angle and horizontal wells, and as a way to reduce barite sag in deviated wells.
Recent studies and field experience indicate that high LSRV is helpful in controlling the invasion of filtrate by creating a high resistance to movement into the formation openings. Since the fluid moves at a very slow rate, viscosity becomes very high, and the depth of invasion of the fluid into the formation is kept shallow. This has been beneficial in protecting the zones from damage as well as reducing differential sticking in these fluids. (See, for example, the article entitled “Drill-In Fluids Improve High Angle Well Production”, Supplement to the Petroleum Engineer International, March 1995).
Lost circulation (loss of whole drilling fluid) is also a severe problem in rotary drilling. Lost circulation occurs when the ECD is much greater than formation pressure. In the extreme case, ECD exceeds formation strength, and the rock fractures. Whether in pores or fractures, the openings in the rock are able to accept and store drilling fluid so that none is returned to surface for recirculation. Whole drilling fluid is lost rapidly downhole and can become an expensive and dangerous problem. Lost circulation can lead to hole instability, stuck drill pipe, and loss of well control. At the least, lost circulation halts drilling operations and requires expensive replacement fluid volume to be used.
In addition to the fluid volume being lost, expensive lost circulation materials (LCM) are required. These are usually fibrous, granular, or flake materials such as cane fibers, wood fibers, cottonseed hulls, nut hulls, mica, cellophane, and many other materials. These LCM are added to the fluid system so that they may be carried into the loss zone and lodge to form a bridge on which other materials may build a seal akin to a filter cake. LCM themselves are damaging to the zones, and because they often must be carried in the drilling fluid to maintain circulation, solids removal is halted and buildup of solids in the mud results.
Methods of correcting lost circulation of drilling fluids by aerating the drilling fluids are set forth in U.S. Pat. No. 2,818,230 (Davis) and U.S. Pat. No. 4,155,410 (Jackson). However, traditional aerated fluids also have disadvantages. Problems with these fluids include hole cleaning, control of formation fluids and corrosion. Standard pumping equipment will experience cavitation, so that expensive, often hard-to-get equipment such as compressors and boosters are required. In addition, such fluids are not recirculateable and must be constantly generated as the drilling proceeds.
In light of the deficiencies of the prior methods, there is still a great need for fluids that can rapidly seal formation fractures and/or inhibit the excessive loss of drilling fluids. In particular, some attractive fluid based systems incorporate aphrons, which are described in U.S. Pat. Nos. 5,881,826, 6,123,159, 6,148,917, 6,156,708, 6,390,208, 6,422,326 and PCT WO 98/36151.